The solid-liquid separator of the prior art is exemplified by a submerged filter system for purification, as shown in FIG. 1. As shown, a raw liquid supply pipe 2 is opened over one side of an aeration tank 1, in which is dipped a filter unit 3. This filter unit 3 is constructed by arranging a plurality of planar filtration membrane modules 5 vertically in a casing 4. These filtration membrane modules 5 are juxtaposed at a constant spacing to form a passage P between the adjacent ones of them. The filtration membrane modules 5 can be formed of a tubular filter medium of ceramics.
To the individual filtration membrane modules 5, there are connected suction tubes 6 which are made to communicate with a permeated liquid suction pipe 7. This permeated liquid suction pipe 7 has its other end connected to a suction pump 8, and a liquid feed pipe 9 is connected to the discharge side of the suction pump 8 and opened above a permeated liquid tank 10.
In the casing 4 of the filter unit 3, there is arranged below the filtration membrane modules 5 a diffuser 11 which is connected via a gas supply pipe 12 to a blower 13. In the bottom portion of the aeration tank 1, there is opened a sludge suction pipe 14 which is connected to a sludge pump 15.
As shown in FIG. 2, each filtration membrane module 5 is composed of a membrane supporting plate 16, a membrane supporting net 17 and a filtration membrane 18. The membrane supporting plate 16 is made of a resin and formed with a permeated liquid passage 19. This permeated liquid passage 19 has its one end opened in the surface of the membrane supporting plate 16 and its other end communicating with the corresponding one of the suction tubes 6.
For the process, a raw liquid 20 is supplied from its supply pipe 2, and an aeration gas 21 such as air containing oxygen is supplied to the diffuser 11 from the blower 13 via the gas supply pipe 12 so that it is injected from the diffuser 11 into the aeration tank 1 to purify the raw liquid 20 biologically.
On the other hand, a purified liquid 22 is subjected to the solid-liquid separation through the filtration membrane modules 5 by applying a vacuum to the permeated liquid passages 19 of the filtration membrane modules 5 via the permeated liquid suction pipe 7 and the suction tubes 6 by the suction pump 8. Then, a solid content such as sludge is trapped by the filtration membranes 18 whereas a permeated liquid 23 having passed through the filtration membranes 18 is sucked via the permeated liquid passages 19 and guided to the permeated liquid tank 10 by way of the suction tubes 6, the permeated liquid suction pipe 7, the suction pump 8 and the liquid feed pipe 9.
In this meanwhile, upward liquid flows are established in the passages P between the individual filtration membrane modules 5 by the air lifting action of the aeration gas 21 so that they act as scavenging flows along the membrane faces to suppress any blocking of the caking layer on the membrane faces.
Incidentally, in case the solid-liquid separator thus far described is to be applied to the treatment of sewage or sludge, it is desired to reduce the power consumption. In order to maintain the membrane scavenging flows, however, a certain rate of aeration is indispensable so that the reduction of the power consumption for the air blower is restricted to some extent. On the other hand, the demand for the aeration per unit filtration area could be reduced by enlarging the vertical length of the membrane modules. From the standpoint of maintenance, however, the desired vertical length is about 1 m at most.